The present invention is in the field of implantable medical devices, and more particularly, is directed to devices useful as a prosthetic intervertebral disc.
The intervertebral disc acts in the spine as a crucial stabilizer, and as a mechanism for force distribution between the vertebral bodies. Without the disc, collapse of the intervertebral space occurs in conjunction with abnormal joint mechanics and premature development of arthritic changes.
Prior art methods of treating injured or diseased discs have included chemical disintegration procedures and surgical excision, often followed by bony fusion to prevent spinal collapse or instability. With excision, no significant regeneration of vertebral tissue occurs. Replacement of an injured disc in an otherwise healthy spine may prevent arthritic changes and may stabilize the spinal segments. In diseased spines, replacement of the disc may reduce the progression of the disease process, and may provide pain relief.
In alternative prior art replacement approaches, discs have been replaced with prostheses composed of artificial materials. The use of purely artificial materials in the spine minimizes the possibility of an immunological response. In addition, such materials permit construction of a structure which can withstand the high and repeated loads seen by the spinal vertebral joints, and can alter the joint mechanics in beneficial ways that biological materials would not tolerate. For example, titanium, (Albrektsson et al. (1981) Acta Ortop. Scan. 52:155-170), acrylic (Cleveland (1955) Marquette Med. Rev. 20:62; Hamby et al.(1959) J. Neurosurg. 16:311), polytetrafluorothylene-carbon fiber (Alitalo (1979) Acta Veterinaria Scandinavica Suppl. 71:1-58), and steel discs (Fenrstrom (1973) Acta Chir. Scand. 4:165-186; have been used to replace the resected disc. Each of these efforts have met with failure due to continued collapse of the disc space or erosion of the metal prosthesis into the surrounding bone.
A prosthetic intervertebral disc has also been constructed from resilient materials such as silicone rubber (e.g., Edeland (1985) J. Biomed Eng. 7:57-62; Schneider et al. (1974) Z. Orthop. 112:1078-1086; Urbaniak et al. (1973) J. Biomed. Mater. Res. Symposium 4:165-186). A disc has also been made from resilient plastic materials to form a bladder as disclosed in U.S. Pat. Nos. 3,875,595 and 4,772,287; however, failure to restore full stability and normal joint biomechanics has prevented success. Porous elastomeric materials as described in U.S. Pat. No. 4,349,921 have failed to recapitulate the normal vertebral body mechanics.
Generally, the replacement of intervertebral tissue with structures consisting of artificial materials has been unsuccessful principally because the opposing vertebral end plates of human and animal joints are fragile. The end plates in the spine will not withstand abrasive interfaces nor variances from normal compliance, which evidently result from the implantation of prior art artificial discs. Additionally, joint forces are multiples of body weight which, in the case of the spine, are typically over a million cycles per year. Thus far, prior art artificial discs have not been soft or durable enough, nor have they been able to be positioned securely enough to withstand such routine forces.
Prostheses, in general, have been devised out of at least some of the constituents of the structures which they are replacing, or out of materials not considered to be immunogenic to the body. For example, Yannas et al. fashioned blood vessel grafts (U.S. Pat. No. 4,350,629), synthetic epidermis (U.S. Pat. No. 4,448,718), and sciatic nerve guides (WO 89/10728; Yannas (1979) Am. Chem. Soc. 16:209) out of collagen and glycosaminoglycans, biochemical components of many body organs. By adjusting the pore size and axes of the pores and fibers comprising these structures, regrowth of natural tissue could be stimulated. Further regrowth has been advanced by seeding of the nerve guide with Schwann cells prior to implantation (see U.S. Pat. No. 4,458,678). However, even with the foregoing technologies which have been applied to the reconstruction of anatomical structures other than intervertebral discs, a structure suitable as a prosthetic disc and constructed from natural materials has not yet been successfully developed.
Accordingly, it is an object of this invention to provide an intervertebral disc replacement or prosthesis.
Another object is to provide an improved disc replacement or prosthesis that does not interfere with normal vertebral segment motion as such interference could lead to a reduced range of motion or to focal concentration of force at other sites within the spinal column or instability of the opposing vertebral bodies, therefore enhancing the chances of progressive arthritic destruction.
Yet another object is to provide an improved disc replacement or prosthesis that is biomechanically able to withstand normal spinal column forces and is able to function at those loads to protect the opposing end plates and stabilize the joints.
Still another object is to provide an improved disc replacement or prosthesis which promotes regrowth of intervertebral disc material and which acts as a scaffold for fibrocartilage infiltration.
A further object is to provide an improved disc replacement or prosthesis which does not evoke an immunologic reaction or aggravate other joint structures.
Still a further object is to provide an improved intervertebral disc replacement or prosthesis which can be easily implanted by standard operative techniques.